In optical communication systems, optical signals are transmitted along an optical communication path, such as an optical fiber. Early optical communication systems deployed a single optical transmitter at a nominal wavelength of light at one end of an optical fiber link and a single optical receiver at the other end of the optical fiber link to detect the incoming optical signals. More recently, wavelength division multiplexed (WDM) systems have been deployed in which multiple wavelengths of light are combined onto a single optical fiber in order to increase the information carrying capacity of the optical communication network.
In a WDM system, multiple optical transmitters feed optical signals to an optical multiplexer that is provided at one end of an optical fiber link and an optical demultiplexer is provided at the other end of the optical fiber link to separate the combined optical signal into its constituent optical signals at corresponding wavelengths of light. Often, however, optical communication network configurations require that given wavelengths of light be selected or “dropped” from the combined optical signal prior to reaching the optical demultiplexer at the termination point of the optical fiber link. In addition, optical signals at the “drop” wavelength of light or other wavelengths of light are often required to be added prior to the termination point of the optical fiber link. Accordingly, optical add/drop multiplexers have been developed that add/drop optical signals at given wavelengths of light, while permitting optical signals at other wavelengths of light to pass to the add/drop or termination points.
A conventional optical add/drop multiplexer is described, for example, in U.S. Pat. No. 6,459,516, the contents of which are incorporated in full by reference herein. This optical add/drop multiplexer flexibly accommodates a relatively large number of added/dropped optical signals or channels. The channels that are added/dropped are fixed, however, and the optical add/drop multiplexer is not remotely reconfigurable.
An alternative optical add/drop multiplexer is a select optical add/drop multiplexer (SOADM), commercially available from CIENA Corporation of Linthicum, Md. As illustrated in FIG. 1, the SOADM receives incoming optical signals through an optical amplifier 110. The optical signals are passed from the optical amplifier 110 to a power splitter or coupler 120, which supplies a first portion of each incoming optical signal to a reconfigurable blocking filter (RBF) 130 and a second portion of each incoming optical signal to a pre-booster amplifier 160 and, subsequently, a router 180. The router 180 separates the second portion of each incoming optical signal into separate channel groups, one of which is passed through a segment of dispersion compensating fiber (DCF) 121, and then to an optical amplifier USA 197. The channel group is then fed to a channel group demultiplexer including a 1×8 splitter 119, which supplies the channel group on each of eight outputs. The splitter 119 is a conventional power splitter, such that the signal strength of each output is attenuated to about ⅛th the power of the input. Channel filters (not illustrated) are coupled to each output of the splitter 119 to select individual channels from each output and supply the demultiplexed channels to corresponding receivers (not illustrated).
Added channels are supplied from transmitters (not illustrated) to an 8×1 combiner 117 through an amplifier 115 and a router 195. At the output of the router 195, the added channel group is passed through an optional segment of DCF 190 and amplified by an amplifier 170. The added channel group is the combined with the channels output from the RBF 130 by a coupler 140, and the resulting WDM signal is output through an amplifier 150.
In operation, the RBF 130 is configured to block the channel group selected by a port 161 of the router 180, while the remaining channel groups pass through. Although non-selected wavelengths of are also supplied to the router 180, no optical demultiplexing elements or optical receivers are provided to sense the non-selected wavelengths of light. The added channels are typically at the same wavelength of light as the blocked channels in order to prevent interference between those optical signals passed through the RBF 130 and those optical signals that are added. Alternatively, the added channels may be different from any of the pass through channels.
Moreover, the RBF 130 may be reconfigured such that a different channel group is blocked. In which case, optical demultiplexers must be added to a different port or slot of the router 180, for example. Since the optical add/drop multiplexers are often deployed in remote locations, service personnel must travel to the optical add/drop multiplexer site(s) and physically attach the channel group optical demultiplexer to a new output port of the router 180.
Alternatively, the RBF 130 may be replaced with a wavelength selective switch (WSS) 210, as illustrated in FIG. 2. WSSs are known components that are coupled to multiple input lines and output lines, and selectively block optical signals on a per wavelength basis. In this instance, the WSS 210 is coupled to input lines 209, 213, and 215, and output lines 222, 225, and 226. The operation of the routers and group demultiplexers is similar to that described above with regard to FIG. 1. However, as illustrated in FIG. 2, additional routers may be provided, each one coupled to a corresponding one of the input lines or output lines. However, the WSS-based optical add/drop multiplexer illustrated in FIG. 2 suffers from disadvantages similar to those described above with regard to FIG. 1. Namely, any reconfiguration of the WSS 210 resulting in a change in the wavelengths of light to be added/dropped requires physically coupling the channel group optical demultiplexers to a different router output port.
ROADMs are the key technology for the next generation of dense wavelength division multiplexing (DWDM) systems. These ROADMs allow for the automated rearrangement of wavelengths of light on the multichannel optical fibers entering and leaving optical network nodes. For a high-degree optical network node, with a degree number of up to 8, for example, directionless ROADMs are preferred because they may route any wavelength of light on any optical fiber (or from any direction) to any given transceiver entirely in the optical domain.
As is described in greater detail herein below, in existing ROADM designs, erbium-doped fiber amplifier (EDFA) arrays with fixed gains or output powers are utilized in order to satisfy a worst case scenario, even though there are only M (e.g. 8 or 16) channels to be dropped for a given modular design. This is not a cost effective solution. Each EDFA is over designed to support the worst case scenario, when all of the wavelengths of light or channels are fully populated. More than 40% of the associated cost is attributed to the pump lasers for the individual EDFAs. In order to simplify the design of the signal distribution modules utilized in directionless ROADM applications, as well as shrink their size and lower their cost, the present invention provides a novel configuration that takes full advantage of type A/type B+ N×M multi-cast switches and the advanced EDFA array design with planar lightwave circuit (PLC)-based tunable pump splitters.